1. Field of the Invention
This invention relates generally to adhesive blends and composite structures and, more particularly, this invention relates to an adhesive blend especially suitable for adhering to propylene polymers and gas barrier materials, and to composite structures incorporating such materials and adhesives.
2. Description of Related Art
The use of propylene polymers together with a gas barrier material, such as an oxygen barrier material, for example, in semi-rigid structures is gaining widespread acceptance in such markets as the food packaging industry. In such applications, propylene polymers provide structural integrity, and the the barrier material reduces the rate and extent of transmission of molecules such as oxygen through the structure. However, propylene polymers will not bond to gas barrier materials without an intervening adhesive. Heretofore, adhesives designed to bond polypropylene to such barrier layers have exhibited a number of disadvantages.
The use of maleic anhydride modified polypropylene in adhesive applications is well known. Blends of modified and unmodified polypropylene and their use as metal coatings are disclosed, for example, in U.S. Pat. No. 3,483,276 assigned to Hercules. Adhesives which can bond polypropylene to a barrier material (such as ethylene vinyl alcohol copolymer, for example) are described in U.S. Pat. No. 4,058,647 assigned to Mitsubishi Petrochemical and in U.S. Pat. No. 4,198,327 assigned to Mitsui Petrochemical. The adhesives of U.S. Pat. Nos. 4,058,647 and 4,198,327 are blends of polypropylene modified with maleic anhydride, a propylene polymer, and a hydrocarbon elastomer or rubber.
However, the use of an elastomer in such an adhesive blend adds to the cost of the blend, and many cause material handling problems. Also, the presence of elastomers in such blends may result in undesirably high contents of extractable materials, which may render the blends unsuitable for use in food packaging applications.
Schmukler et al U.S. Pat. Nos. 4,472,555 and 4,409,364 disclose adhesive blends which provide adhesion to both polypropylene and polar substrates. These blends are mixtures of modified polyethylene, polyethylene and polypropylene.
Liu U.S. Pat. No. 4,510,286 assigned to Hercules discloses a bonding resin composition which comprises maleic anhydride modified polypropylene, a sequentially polymerized propylene/ethylene copolymer having an ethylene content of 15-30 wt.% and a propylene polymer or propylene/ethylene copolymer having an ethylene content of 0-14 wt.%.
Adhesive blends made according to the Schmukler et al or Liu patents may not provide adequate adhesion to polar substrates (such as some gas barrier materials) for some applications.